Micro-electro-mechanical systems (MEMS) devices that change the capacitance of an electronic capacitor have great utility in radio frequency (RF) applications because they provide ability to change the capacitance of a circuit element, thus changing the electronic properties of the circuit. A number of efforts have been made to build miniaturized MEMS capacitive devices that switch between two states of capacitance utilizing thin film manufacturing techniques that are similar to the semiconductor manufacturing process. These devices share a common design where a first conductive plate is actuated to move from a first stable position to a second stable position, closer to a second plate, forming a parallel plate capacitor. The second metal carries the electrical signal. Because the device has only two stable states, and thus two capacitances, it is known as a two-state capacitive switch or simply a capacitive switch. Capacitive switches designed and made accordingly suffer from poor performance and are not capable of switching signals having large power.
The performance is limited by several factors. First, the devices are manufactured on a silicon substrate. Silicon is “lossy” meaning that it is known to absorb energy from radio waves, and it is also slightly conductive, thus allowing some leakage of radio energy through the substrate. Thus, capacitive switches made on silicon exhibit poor isolation and high loss. In addition, these devices are typically made with the first movable plate element constructed of a thin membrane of metal, typically less than 1 micrometer thick, which is pulled towards the second plate via electrostatic attraction. A thin dielectric coating on the second plate prevents direct short circuiting between the two plates, allowing the two plates to form a capacitor. To produce enough electrostatic actuation, a voltage must be formed between the two plates, typically 40 volts or more. As a result, the plates must be manufactured close together (typically a few microns separation), and the membrane must be very thin and easy to move. When actuation of the membrane occurs, the voltage must be maintained in order for the switch to stay latched in its actuated state.
MEMS capacitive switches for high power applications are difficult to design using conventional silicon technology. Silicon MEMS devices (and their close variants, such as electro-formed metal devices) generally result in closely spaced, fragile elements. Most switches use electrostatic actuation to move the switch arm into contact with the mating electrical contact. This can only be achieved if the switch arm is close to the actuating mechanism, and if the actuation force is small. These requirements may be acceptable for low power (<1 W) applications where close proximity has little deleterious effect. However, for high power applications, it is unacceptable. Power coupling across the small gap between conductors is appreciable at high power. Self charging occurs at high power resulting in self actuating switches (the “hot switch” effect), and high power applications require that high current be passed through the conducting elements, which can destroy the thin membranes used in typical electrostatic RF MEMS devices.
Silicon devices can result in power losses through a lossy substrate, which for high power applications can result in heating and device failure, as well as degradation in performance. Furthermore, nearly all micro-machined devices must eventually be placed in a protective housing so that electrical connections can be made to the devices, and to protect the devices. This is troublesome for RF MEMS devices because they are fragile and because the electrical connections have an unknown effect on the device impedance.